1. Field of the Invention
The present invention relates to a printed circuit board of two or more layers, and electronic components.
2. Background of the Invention
In recent years, as electronic apparatuses have become smaller and denser, multilayer printed circuit boards being inexpensive and in which semiconductor chips such as LSIs may be mounted at a high density have been strongly demanded not only in the field of industrial apparatuses but also in the field of consumer apparatuses. In such multilayer printed circuit boards, it is important that fine-pitch interconnecting patterns on the layers are electrically connected between the layers with a high degree of reliability.
With respect to this demand, it is extremely difficult to satisfy it by use of conventional printed. circuit board manufacturing methods using drilling, etching of copper-coated laminations or plating, so that printed circuit boards having new structures are being developed.
Current representative examples of high-density printed circuit boards include the following:
The first example is a method called a built-up process in which a conventional double-sided or multilayer printed circuit board is used as the core board and insulating layers having smaller via holes and wiring layers being finer are laminated thereon (Susumu Honda "Current State and Problems of Built-up Multilayer Printed Circuit Board Technology", Magazine of Japan Institute for Interconnecting and Packaging Electronic Circuits, Vol. 11, No. 7, pp.462-468 (1996)). According to this method, small holes for smaller via holes are formed in the insulating layers by photoetching or chemical etching by use of a photosensitive insulating material or a chemically etchable material. Recently, methods have been developed which use plasma or a laser to form the holes in the insulating layers. When a laser is used, it is unnecessary for the material to have photosensitivity or chemically etchable property, so that the insulative material may be selected from a wider range.
The second example is a stacking process in which copper foils are laminated on a prepreg with holes filled with a conductive paste (Hiroyuki Okano, "Resin Multilayer Circuit Board with All Layers Having IVH Structure", 1995 Microelectronics Symposium, p.163 (1995)) (generally, this type of circuit board is frequently classified as a built-up multilayer printed circuit board). Researches have been carried out on a circuit board using instead of a prepreg a film having an adhesive (Keiichi Takenouchi et al. "Development of Polyimide Multilayer Circuit Board", Papers Presented at the Tenth Lecture Meeting on Interconnecting and Packaging Electronic Circuits, pp.81-82 (1996)).
The third example is a method using a thin film multilayer and is similar to the built-up process described as the first example. This is a thin film multilayer circuit board in which a conventional ceramic multilayer circuit board is used as the core board and on surfaces thereof, inorganic or organic insulating layers and conductive wiring layers patterned by plating and photoetching, etc. are laminated. At present, this method, which uses thin film technology, is the method by which the densest printed circuit boards are manufactured. For the insulating layers, photosensitive polyimide is most frequently used.
However, these printed circuit board manufacturing methods have defects.
According to the built-up process, it is difficult to obtain denser printed circuit boards because a conventional low-density glass epoxy multilayer circuit board is used as the core of the inner layer. In addition, in order to obtain denser printed circuit boards, it is necessary to increase the number of insulating and wiring layers being built up, so that it is technically difficult to flatten the surfaces of printed circuit boards, or the cost increases.
According to the stacking process, it is difficult to form at a low cost the small holes in a prepreg or a film (base material) of a thickness for higher density.
According to the thin film multilayer method which uses a thin film process, the cost is high.